1. Field of the Invention
This invention relates generally to corrosion monitoring systems and methods and, more particularly, to corrosion monitoring systems and methods for use in steam generators to monitor the corrosive effects of chemical cleaning of steam generators.
2. Prior Art
Certain nuclear power stations using pressurized water reactor technology utilize a large heat exchanger, known as a steam generator, to transfer heat from the reactor coolant system (i.e. heat generated during nuclear fission) to a secondary system. The heat transfer results in the boiling of secondary steam feedwater, which produces steam to drive the plant's turbine, which in turn powers the electric generator. The steam is then condensed and returned to the steam generator in a continuous recirculation loop. The secondary feedwater contains trace quantities of contaminants, primarily iron oxide, which tend to plate out inside the steam generator, eventually resulting in flow blockage, corrosion, and/or loss of heat transfer capability. An emerging mitigation method for removing such contaminants involves the use of a chemical solvent, usually during unit shutdown, to dissolve the contaminants.
One of the concerns with chemical cleaning is the potential for corrosion of steam generator internal components by the cleaning solvent. There are a number of steam generator internal components which are potentially susceptible to chemical cleaning corrosion, primarily carbon steel components and welds and, more particularly, those which are influenced by the galvanic fields of the more noble metals (e.g. Inconel 600, a Ni-Cr-Fe alloy) generally used for steam generator tubing. With the additional criteria that tube support structures (usually constructed of carbon steel) have very low corrosion allowances, the corrosion of tube support structures frequently becomes the controlling factor in chemical cleaning. Tube support structures are components of the steam generator which provide lateral support to the primary 10 water tubes in order to prevent excessive vibration. Actual tube support structure corrosion after cleaning cannot be measured due to accessibility problems.
There are basically two types of corrosion of concern in chemical cleaning: free corrosion due to base metal contact with the cleaning solvent, and galvanic corrosion, which is induced by metals such as Inconel 600 in the proximity of a base metal such as carbon steel. Total corrosion is the sum of free corrosion and galvanic corrosion. In order to monitor corrosion caused by chemical cleaning operations, and to assure that excessive base metal is not removed along with the contaminants, corrosion monitoring systems can be utilized during cleaning.
There are generally two prior art methods to measure corrosion in a steam generator: (1) coupons and (2) electronic measurements. "Coupons" are simply sample pieces of base metal which are inserted into the steam generator during cleaning operations. At a desired time, the coupons are removed to measure actual corrosion which took place inside the steam generator. Coupons can provide either free or total corrosion measurements, depending on the location of the coupons. By coupling Inconel 600 to base metal coupons, with appropriate area ratios maintained to approximate the effective area ratios of the two metals in the steam generator, one can obtain an approximation of total corrosion. The correlation of the corrosion of the coupon to actual tube support structure corrosion depends upon how well the geometric similarity within the steam generator is approximated by the coupons Without the ability to perform full scale testing, or to measure actual tube support structure corrosion in field applications, the accuracy of the correlation is uncertain. Based upon the relatively low allowable tube support structure corrosion (as low as three mils total corrosion), the need exists for maximum assurance that corrosion correlations are accurate. Premature total corrosion correlations in excess of the allowable may result in corresponding premature replacement of the steam generator at great expense. Due to the unknown accuracy of coupon measurements caused by uncertain geometric relationships, the use of galvanic coupons to approximate total corrosion has been suspect. Additionally, corrosion coupons can only be checked after the cleaning process is complete or by interrupting the process, and on-line monitoring is desirable to assure the process remains under control, with low corrosion rates.
Electronic measurement of corrosion ca be utilized to monitor corrosion on-line during the chemical cleaning process. Linear polarization ("LP") probes provide an indication of free corrosion, while zero resistance ammetry ("ZRA") probes provide an indication of galvanic corrosion. The accuracy of electronic readings is also suspect due to the influence of the relative configurations within the steam generator as compared to the conditions surrounding the probes. This influence is due to both the galvanic fields created as a result of geometry and to the local chemistry conditions (e.g. in the crevices between the primary water tubes and the tube support structure).
Previous corrosion monitoring schemes used in the field utilized LP and ZRA probes located inside the few access points available in steam generators. The required proximity to the tube/tube support structure, and the effects on measured versus actual corrosion were uncertain, although there was fairly good correlation between electronic and coupon corrosion. Additionally, some utilities placed large scale tube support structure mockups in side stream monitors which took solvent from the steam generator during cleaning operations and recirculated the solvent through the monitor to attain more realistic geometry. The mockups could be accessed for actual measurements to correlate with the electronic indications, and to better correlate to actual tube support structure/tube geometry. However, concern existed that the chemical environment within the mockup would not be identical to that in the steam generator due to temperature losses in the recirculation system and the absence of the relatively short-lived intermediate species which occur during sludge dissolution within the steam generator (which accounts for a large percentage of the total corrosion). The intermediate species is believed to react to become a less aggressive species before reaching the mockup via recirculation. Testing performed by Electric Power Research Institute utilized large scale configurations of tube support geometry to assess the importance of geometry on corrosion monitoring. The results indicated that certain aspects of the geometry were critical, including the length of tubing on either side of the tube support structure.